1. Field of the Invention
The present invention relates to a signal processing apparatus for making a self-diagnostic check of the desensitization of a transmitted/received signal of radar in the signal processing of radio radar.
2. Description of the Related Art
Recently, the travel control technology for vehicles has been remarkably developed, and various systems of supporting the driving operation of a driver have been developed. In these systems, a radio radar unit loaded into a vehicle detects the presence of obstacles around a vehicle, the data of the detection result is processed, and a computer determines how to control the vehicle, for example, applying the brakes, wheeling left or right, etc. The driver controls the vehicle based on the determination of the computer. It is also planned to apply these systems to a future automatic driving system.
A conventional apparatus for detecting a fault in a radio radar unit can be the apparatus described in the patent document 1. The apparatus described in the patent document 1 has a modulator for performing modulation for detection of a fault, detects a modulation component in reflected waves, thereby determining a fault.
[Patent Document 1]
Japanese Patent Application Laid-open No.10-62525
FIG. 1 shows an example of using radio radar.
A radio radar unit is loaded into a vehicle, etc., measures a preceding car, etc., and performs control such as homing guidance, immediate brakes, etc. on the vehicle according to the information. A radio radar unit loaded into a vehicle is often used in the FM-CW system to obtain data of a preceding car, etc. The CW system can also be used to obtain self-diagnostic check data.
FIG. 2 shows the concept of the radio radar.
Radio waves are emitted from a radio wave transmitter which is an antenna, and reflected radio waves are received by a radio wave receiver which is also an antenna. In the signal processing, the propagation time of radio waves and a Doppler shift are measured, and they are converted into a distance and a velocity.
FIGS. 3 through 13 show the concept of the principle of the operation of radio radar and the conventional problems.
FIG. 3 is a graph showing a horizontal axis indicating time and a vertical axis indicating frequency. The triangular wave in the solid line indicates the frequency of emitted radio waves. The triangular wave in the chain line indicates received radio waves from an object standing still at a distance D, and it is apparent that there occurs the displacement by an emission time. Obtaining the difference between a transmitted wave and a received wave proportional to a propagation time is simpler than direct measurement of a propagation time.
As shown in FIG. 4, a frequency difference fr signal (referred to as a beat signal) between the transmission and the reception is generated, the beat signal is sampled in the rise section and the fall section of a triangular wave, a Fourier transform is performed on each of them, thereby obtaining electric power. As a result, there occurs a peak in the frequency difference fr, the peak is detected, and the distance D is obtained from the fr.
FIG. 5 shows the state of the received radio wave from an object having a traveling velocity V at a distance D. There occur the displacement by the propagation time and the frequency displacement fd of the Doppler shift. In the rise section and the fall section of a triangular wave, a beat signal is sampled, and each of the beat signals is Fourier-transformed, thereby obtaining electric power. As a result, a frequency value peak fup is obtained by subtracting the displacement fd by the Doppler from the frequency difference fr generated depending on the propagation time in the rise section, and a frequency value peak fdown is obtained by adding the frequency difference fr generated depending on the propagation time in the fall section to the displacement fd by the Doppler shift. The frequency difference fr is obtained depending on the propagation time in the adding operation, etc. of the fup and the fdown, the displacement fd by the Doppler shift is obtained in the subtracting operation, etc. of the fup and the fdown, and the obtained values are multiplied by a constant to convert them into distance and velocity values.
FIG. 6 shows the principle of the operation of the CW radio radar.
In the CW system, only the velocity is measured, and those having the same velocity cannot be separated. Unlike the FM-CW system, no frequency modulation is performed, and radio waves at a constant frequency are transmitted. When the traveling velocity is different from the velocity of the vehicle of the present apparatus, a Doppler effect occurs. Therefore, the received signal is mixed with the transmitted signal, and the high frequency component is filtered. Then, only the Doppler signal at a low frequency remains, and the signal is Fourier-transformed. Then, the received power of the reflected wave from an object indicating the difference from the velocity of the vehicle of the observer is obtained. Based on the result, a peak in the frequency range corresponding to the velocity of the vehicle of the observer having an opposite sign is searched, thereby collectively obtaining the received power of the reflected waves from the road structure and the road.
FIG. 7 shows an example of a time chart of the FM-CW mode and the CW mode of the radar loaded into a vehicle.
Since the FM-CW mode detects a preceding car, etc., the operation is continuously performed except when the operation is performed in the CW mode. Since the CW mode is used in the self-diagnostic check of the desensitization, the operation is performed once while operation in the FM-CW mode is performed several times.
Conventionally, the self-diagnostic check of the desensitization has been made based on the level of the received power of the CW signal from the road structure and the road. It is certain that, as shown in FIG. 8, the reflection is mostly received from the road when there are substantially no road structures, and almost the same received power is obtained from any directions as shown in FIG. 9.
FIG. 10 shows a schematic diagram of the histogram of an average value of received results of the received power of a predetermined number of times.
As shown in FIGS. 9 and 10, normal received power can be clearly separated from the received power of desensitization. In a process, the data in the CW mode of several times is accumulated, the entire measurement data is averaged regardless of the direction, the average value is compared with a threshold, and it is determined whether or not the average value is larger than the threshold (hereinafter the processing performed using a threshold as display above is referred to as “slice processing”). It is checked whether or not the value equal to or smaller than the threshold continues in time. If yes, the desensitization is announced.
However, if there is a road structure as shown in FIG. 11, the reflection from the road structure is received from both sides even though the desensitization is detected as shown in FIG. 12. Therefore, if an average value is obtained regardless of the direction, the average value obtained when there are no road structures as indicated by the histogram shown in FIG. 13 cannot be discriminated from the average value obtained when there are a number of road structures with the desensitization, thereby presenting the problem that the desensitization cannot be detected in a self-diagnostic check.